In controlling a mechanical system, it may be necessary to sense the relative motion or position of a moving member. To accomplish this task, optical sensors have been developed for scanning a light reflective pattern applied to the surface of the member being monitored. Light reflected from the surface is modulated by the pattern, producing an optical signal that encodes the motion of the surface relative to the sensor and thus defines its position. By using multiple reflected light channels, each conveying the status (1 or 0) of one bit of a digitally encoded signal, the optical sensor can directly determine either rotary or linear position of the surface, with a resolution dependent only on the number of channels (or bits) employed.
Prior art optical sensors have been constructed using bundles of micro-optical fibers for splitting and coupling light reflected from a moving surface. Construction of an optical sensor head using the previously known techniques is labor-intensive, requiring many man-hours of skilled, tedious work with the bundles of micro-optical fibers. In one design, 108 micro-optical fibers are used in the bundle for each channel of encoded data. The cost of constructing optical sensors in production quantities using such techniques, particularly with more than three channels, is too high to be commercially acceptable.
Use of a waveguide for constructing an optical sensor has generally been considered impractical in the past, due to the newness of waveguide technology and the relatively high losses associated with branching circuits formed in a waveguide. Recently, however, a new technique has been developed for producing high-silica channel waveguides having an additional low refractive index layer at the branching junction between two waveguide channels, which reduces radiation loss. In a paper entitled, "Low Radiation Loss Branching/Combining of Optical Circuits Using High-Silica Channel Waveguides," published in the Journal of Light Wave Technology, Vol. LT-4, No. 12, Dec. 1986, S. Sumida, K. Magari, and M. Kawachi explain this technique and indicate that it can be used to make a wavelength division multiplexer/demultiplexer.
A wavelength division multiplexer typically includes multiple sources of light, each operating at a discrete, different frequency. Light from the different sources is modulated to convey information or data in a plurality of channels, and the multiplexer combines the different wavelengths of modulated light into a single channel. Conversely, an optical wavelength demultiplexer splits light comprising different wavelengths into separate channels. Each channel is connected to a light detector, such as an avalanche of PIN photodiode, which is sensitive to that wavelength, and which produces an electrical signal corresponding to the data encoded on that channel.
There is clearly an advantage in multiplexing data, since a multiplexed optical sensor can convey data for all channels scanned by the sensor on a single optical fiber. The bundled micro-optical fibers from the prior art optical sensor head could of course be separately connected to the individual inputs of the waveguide multiplexer described in the above reference; however, there would be no reduction in fabrication costs of the sensor; in fact, the overall costs of the system would be substantially higher.
Accordingly, it is an object of this invention to integrate an optical sensor and a wavelength division multiplexer in a single device. It is a further object to reduce the production costs for manufacturing an optical sensor having a plurality of channels for sensing motion and position of a surface. These and other objects and advantages of the present invention will be apparent from the attached drawings and the Description of the Preferred Embodiments that follows.